Customarily, the magnetic powder used for magnetic recording media, represented by audio and video tapes, has been predominantly of .gamma.-iron oxide and of Co-.gamma.-iron oxide.
In recent years, high density magnetic recording media have been desired.
It has been known that ferromagnetic metal particles composed primarily of metallic iron or an alloy thereof obtained by the catalytic reduction of a powder mainly comprising iron oxyhydroxide or iron oxide with a reducing gas possess a high coercive force and high saturation magnetization. Thus, these particles have been investigated as high density magnetic recording media.
High density magnetic recording media for audio and video applications are demanded to produce higher outputs and less noises in broad frequency bands.
Factors demanded for the magnetic particles for this purpose are such that the size of the particles should be more minute, their shape be acicular or prismatic, they be more stable to oxidation, and they should maintain high saturation magnetization after environmental deterioration or corrosion tests.
Several processes have been proposed to date to obtain powdery ferromagnetic metal particles composed of iron or primarily of iron by heating and reducing iron or metallic compounds composed mainly of iron as a starting material in a reducing atmosphere.
(1) For example, Japanese Patent Laid-Open No. 134858/1977 discloses a process for producing ferromagnetic metallic iron comprising doping iron oxyhydroxide or oxide with specific elements to form a starting material, attaching the hydroxide of Si or Al to the starting material and subsequently heating and reducing the resulting material.
(2) Japanese Patent Laid-Open Nos. 122663/1979 and 2664/1979 (GB 2016526) describe processes for producing ferromagnetic metallic iron by heating and reducing iron oxyhydroxide or oxide with or without specific elements doped, which has been attached with the hydroxide of Zn, Cr, Cu, Co, Ni, Mn or Sb.
(3) Japanese Patent Laid-Open No. 173209/1984 teaches a process for producing ferromagnetic metal particles comprising neutralizing aqueous solutions containing a compound of Mg, Ti, Mn, Co, Ni, Cu or Zn and ferrous ions (Fe.sup.2+) individually to form the hydroxides, blowing thereto air to convert them spinel-type compounds, attaching these compounds onto iron oxyhydroxide particles, and reducing the resulting materials.
(4) The iron or a metal mainly comprising iron obtained by heating and reduction tends to be oxidized or burned in the air when left as it is, and hence its magnetic property is degraded time-dependently with the progress of oxidation. Then, Japanese Patent Laid-Open No. 55503/1981 discloses a process for producing metallic magnetic powder excellent in corrosion resistance by further forming oxide films on the surface of the aforesaid metal particles in the atmosphere of low oxygen concentrations.
As described above, the demand for ferromagnetic metal particles as a magnetic powder for high density recording is such that the particles be more minute without impairing high saturation magnetization.
Thus, many further difficulties are left, including imparting the stability to oxidation to the minute particles and controlling the morphology of the minute particles by preventing their sintering.
The following problems are involved in the prior art processes by way of example.
(1) In the process for producing ferromagnetic metal particles by the attachment of the hydroxide of Al or Si to iron oxyhydroxide, etc. followed by reduction, the more minute the particles, the more serious is their sintering, so that it is difficult to maintain their morphology.
Further, even if oxide films are formed on the surface of the metal particles by gradual oxidation, this hardly has the effect of preventing the progress of oxidation.
(2) In the process of producing ferromagnetic metal particles comprising attaching a metal hydroxide obtained by neutralizing an aqueous solution of Zn, Cr, Cu, Co, Ni, Mn or Sb to iron oxyhydroxide or oxide with or without specific elements doped and reducing the resulting material, the metal hydroxide obtained by the neutralization have high crystallization rate so that it grows to form crystalline particles with the same or larger size than that of the iron oxyhydroxide particles. As a result, said metal hydroxide particles can hardly attach to the iron oxyhydroxide particles. Consequently, the metal hydroxide particles and the crystalline iron oxyhydroxide particles are present independently, forming a simple mixture.
On heating and reducing such a mixture, it is often observed that the reduced metal particles no longer have the morphology of the original particles but they collapse or sinter to each other. Further, in these particles, it is also observed that metal particles or oxide particles derived from the reduction of the metal hydroxide particles are admixed.
Further, the formation of oxide films by gradual oxidation of the reduced ferromagnetic metallic iron particles with a small amount of oxygen does little to prevent the progress of oxidation of the particles.
(3) In the process of obtaining ferromagnetic metal particles by neutralizing Mg, Ti, Mn, Co, Ni, Cu or Zn ions and ferrous ions to form the hydroxides, oxidizing them with air to convert them into a spinel compound, and covering/attaching the compound onto acicular iron oxyhydroxide followed by heating and reduction, there has been such a disadvantage that if the constituent elements in the spinel compound are 6 atom % or more per Fe atom in the iron oxyhydroxide, the degree of the covering/attaching reaches saturation and the spinel compound is isolated from the surface of the iron oxyhydroxide in the step of covering/attaching the particles of the spinel compound onto the iron oxyhydroxide particles.
Further, in this case also, the formation of oxide films by gradual oxidation of the reduced ferromagnetic metallic iron particles with a small amount of oxygen has little effects of preventing the progress of oxidation.